The present invention relates to a liquid crystal display device for the display of various images, etc. and particularly to a liquid crystal display device high in both response speed and transmissivity.
Liquid crystal display devices are in wide use as display devices capable of being reduced in weight, size and thickness. Above all, an active matrix type liquid crystal display device of a twisted nematic mode (TN mode) is widely known as a display device low in both driving voltage and power consumption, high in contrast and capable of attaining a high definition.
A general TN mode liquid crystal display device of this type is constructed such that two glass substrates having polarizing plates, transparent electrodes and orienting films are disposed spacedly in an opposed relation to each other so as to be different 90.degree. in orienting directions of the respective orienting films, with a nematic liquid crystal being disposed so as to permit 90.degree. twisted arrangement thereof.
Recently, however, the field angle dependence of this type of a TN mode liquid crystal display device has been posing a problem. FIG. 7 shows a general field angle dependence of a TN mode liquid crystal display device. In the same figure, the angle (50.degree.) represents an inclination from the normal direction and the hatched area represents an area of 10 or more in contrast (CR). According to FIG. 7, the TN mode liquid crystal display device is superior in visibility in the transverse direction, but it is apparent that the visibility in the vertical direction, especially the visibility from above, is extremely poor.
In view of the above point, the applicant in the present case has previously filed a liquid crystal display device capable of solving the above-mentioned problem, as Japanese Patent Laid-Open No. Hei 7(1995)-306276.
According to the construction of the patent application, it is not that a liquid crystal driving electrode is provided on each of upper and lower substrates which sandwich a liquid crystal therebetween, but two types of linear electrodes 12 . . . and 13 . . . of different poles are formed spacedly from each other on only the lower substrate 11 shown in FIG. 8, while no electrode is formed on the upper substrate 10 as in FIG. 9. With a voltage applied, liquid crystal molecules 36 can be oriented in the direction of a lateral electric field generated between the linear electrodes 12 and 13.
More specifically, the linear electrodes 12 are connected together at a base line portion 14 to constitute a comb teeth-like electrode 16, while the linear electrodes 13 are connected together at a base line portion 15 to constitute a comb teeth-like electrode 17, in such a manner that the linear electrodes 12 and 13 of the comb teeth-like electrodes 16 and 17 are in an alternately adjacent and meshed state but not in contact with each other. A power supply 18 and a switching element 19 are connected to the base line portions 14 and 15.
As shown in FIG. 10A, an orienting film is formed on the liquid crystal side of the upper substrate 10, to which is applied an orienting treatment to arrange liquid crystal molecules 36 in .beta. direction, while on the liquid crystal side of the lower substrate 11 is formed an orienting film, to which is applied an orienting treatment to arrange liquid crystal molecules 36 in .gamma. direction parallel to the .beta. direction. To the substrate 10 is laminated a polarizing plate having a polarizing direction in the .beta. direction in FIG. 10A, while to the substrate 11 is laminated a polarizing plate having a polarizing direction in the a direction in the same figure.
According to the above construction, when no voltage is applied between the linear electrodes 12 and 13, the liquid crystal molecules are oriented homogeneously in the same direction, as shown in FIGS. 10A and 10B. Light which has passed through the lower substrate 11 in this state has been polarized in the .alpha. direction by the polarizing plate on the lower substrate and it passes through the layer of liquid crystal molecules 36 and reaches the polarizing plate of the different polarizing direction .beta. on the upper substrate 10, so that it is cut off by that polarizing plate. Thus, the light does not pass through the liquid crystal display device, which device therefore assumes a dark state.
Next, when a voltage is applied between the linear electrodes 12 and 13, the closer to the lower substrate 11 the liquid crystal molecules 36, the more perpendicularly are changed their orienting direction relative to the longitudinal direction of the linear electrodes 12. To be more specific, by a lateral electric field generated by the linear electrodes 12 and 13, there are generated electric lines of force perpendicular to the longitudinal direction of those linear electrodes, resulting in that the liquid crystal molecules 36 oriented with their longitudinal direction facing the .gamma. direction by the orienting film formed on the lower substrate 11 are changed their orienting direction into the .alpha. direction perpendicular to the .gamma. direction by a restricting force of the electric field stronger than the restricting force of the orienting film.
Thus, the application of a voltage between the linear electrodes 12 and 13 causes a 90.degree. twisted orientation, as shown in FIGS. 11A and 11B. In this state, the polarized light which has passed through the lower substrate 11 and has been polarized in the .alpha. direction is changed its polarizing direction by the twisted liquid crystal molecules 36, so that it can now pass through the upper substrate 10 having a polarizing plate in the .beta. direction different from the .alpha. direction and hence the liquid crystal display device assumes a bright state.
FIGS. 12 and 13 show a structure which is assumed to be available when the liquid crystal display device provided with the linear electrodes 12 and 13 of the above structure is applied to an actual active matrix liquid crystal drive circuit.
In the structure shown in FIGS. 12 and 13, a gate electrode 21 and first linear electrodes 22,22, as metallic electrodes, are formed spacedly in parallel with each other on a transparent substrate 20 such as a glass substrate for example, and a gate insulating film 24 is formed so as to cover those electrodes. Further, a source electrode 27 and a drain electrode 28 are formed so as to sandwich a semiconductor film 26 from both right and left sides on the gate insulating film 24 at a position just above the gate electrode 21. On the gate insulating film 24 and between the first linear electrodes 22 and 22 is formed a second linear electrode 29 as a metallic electrode. A planar structure of the structure shown in FIG. 12 is illustrated in FIG. 13, in which gate lines 30 and signal lines 31 are formed in matrix shape on the transparent substrate 20. Further, at a corner portion of each of areas surrounded with the gate lines 30 and the signal lines 31 is formed a gate electrode 21 by drawing out from the associated gate line 30, and the second linear electrode 29 is connected to the drain electrode 28 through a base line portion 33. The first linear electrodes 22 are arranged planarly so as to sandwich both sides of the second linear electrode 29 therebetween and are connected with each other through a base line portion 34. The base line portions 33 and 34 are superimposed one on the other through the gate insulating film 24 shown in FIG. 12 to ensure capacitance in this portion.
In the above structure, a lateral electric field is exerted so as to form electric lines of force in the directions indicated by arrows in FIGS. 12 and 13, so that the liquid crystal molecules 36 are oriented as in FIG. 12 in accordance with the lateral electric field.
Although the above structure of the liquid crystal display device has a merit of the field angle being extremely wide, it involves the problem that the opening ratio is small and the response speed is low.
The problem that the opening ratio is small can be coped with by adjusting the brightness of the back light provided in the liquid crystal display device, and the problem that the response speed is low can be coped with by increasing the applied voltage, taking into account the property of the liquid crystal that its response speed depends on the electric field intensity and the stronger the electric field, the higher the response speed.
However, since the above remedial measures premise that the power consumption is sacrificed, there still remain the problem that the liquid crystal device cannot be diminished its power consumption.
According to another countermeasure, the distance among the linear electrodes 22, 29 and 22 is made short to increase the electric field intensity and thereby improve the response speed. However, this method causes an increase in the number of electrodes, and the electrode width is restricted to the level of the wiring technique and cannot be made smaller than a certain value, resulting in a decrease of the opening ratio. Thus, also in this method there arises the necessity of making the back light brighter to compensate for the decrease of the opening ratio, and this leads to the problem that it is impossible to decrease the power consumption.
In liquid crystal display, when viewed from the standpoint of human engineering, the human eyes tend to easily recognize a low response speed in half tone and bright display. Therefore, a high-speed response of liquid crystal is desired in half tone and bright display.